Automatic calibrating and measuring system

ABSTRACT

A system with a test position for measuring the amplitude (or phase) of an incoming signal wave of predetermined frequency is repetitively switched over to a calibrating position in which the system receives a reference oscillation of constant frequency and phase for automatically readjusting its circuits, if necessary. If the input signal disappears or drops below a certain minimum level, a sensing circuit reduces the rate of switchover or arrests the system in test position to reduce wear of the relay contacts employed therefor. A random or quasi-random change in that rate may be utilized to prevent false indications due to the stroboscopic effect of periodic switchover in the presence of a modulating frequency close to the switching rate.

United States Patent 1 Bayer Apr. 3, 1973 [54] AUTOMATIC CALIBRATING AND3,196,348 7/1965 Parker ..324 120 MEASURING SYSTEM PrimaryExaminer-Rudolph V. Rolinec [7 51 Inventor. Herbert Bayer,Reutlmgen,Germany Assistant Examiner Emest R Kaflsen [73] Assignee:Wandel u. Goltermann, Reutlingen, Attorney-Karl F. Ross Germany [22]Filed: June 7, 1971 [57] ABSTRACT A system with attest position formeasuring the am- [211 App! l50438 plitude (or phase) of an incomingsignal wave of predetermined frequency is repetitively switched over toa calibrating position in which the system receives a [30] Fme'gnApphcahon Pnomy Data reference oscillation of constant frequency andphase June 24, 1970 Germany ..P 20 31 045.6 for automaticallyrcadjusting its circuits, if necessary- If the input signal disappearsor drops below a certain [52] US. Cl .324/130, 324/74 minim m le l, ansing circuit reduces the rate of [51] Int. Cl. ..G01r 1/02, 6011' 1/38switchover r arre h y em in test position to [58] Field of Search..324/130, 120, 74; 330/9 r e wear of h relay contacts employedtherefor. A random or quasi-random change in that rate may be [56]References Cited utilized to prevent false indications due to thestroboscopic effect of periodic switchover in the presence of UNITEDSTATES PATENTS a modulating frequency close to the switching rate.

3,105,230 9/1963 Maclntyre ..324/l30 8 Claims, 6 Drawing Figures 5/9 Tagar THRESHOLD i m 505 506 FR 574- gggg acre-craze 7 SHEET 1 2 519 T l aI THRESHOLD 505 I 70 50 e r 6 y l kwfflfy 57* I SELECTOR ira-(r02 520 UI l I 51 -d, i I 70 50,5 rims-sum I I i 507 4 I j I T {0 FIG 2 l Y F F(a F r r T (c r H r WHITE-NOISE Fl 6 074 GEN m? 5 Herbs Bayer [8 0-' 5INVENTOR.

Attor y PATENTEUAPR3 I975 SHEET 2 OF 2 Herbert Bayer INVENTOR.

Attorney AUTOMATIC CALIBRATING AND NIEASURING SYSTEM My presentinvention relates to an automatic calibrating and measuring system ofthe general type described in my prior US. Pat. Nos. 3,461,385 and3,486,l l2.

In such a system, an incoming signal wave of predetermined frequency isintermittently fed to a selector circuit which passes that frequency toa detector to energize an amplitude indicator, such as a voltmeter;between samplings of this signal wave the system is switched to acalibrating position in which a reference oscillation travels over thesame path to a sensing circuit controlling that path to adjust itsimpedance characteristic, if necessary. An analogous system may be usedto measure phase rather than amplitude, with substitution of a phasecomparator for the detector; the phase comparator may be preceded by anadjustable phase shifter under the control of the sensing circuit.Reference in this connection may also be made to commonly ownedapplication Ser. No. 737,544, filed by me jointly with Peter Harzer,Gunther Hoffmann and Bemd Zabel on June 17, 1968, now patent U.S. Pat.No. 3,584,295, which discloses a system of this type including a digitalattenuator and a digital phase shifter whose settings can be readjustedduring a recurrent calibration interval. As will be apparent from thisearlier disclosure, the two signal parameters (amplitude and phase)could be measured simultaneously in the same system.

The switchover of the selector network between its calibrating and testpositions occurs relatively frequently, e.g., at a rate of 1 cycle persecond. Thus, testing intervals of 0.9-second duration may alternatewith calibration intervals of (ll-second duration. This alternationgenerally continues also during periods of idleness, i.e., in theabsence of the signal to be monitored; as a result, the contacts of theswitching relays are subjected to rather rapid wear.

It is, therefore, an important object of my present invention to providea system of this general type in which a rapid switchover betweencalibrating and test positions occurs only during actual measurements,thereby increasing the service life of the circuit elements involved.

Occasionally, with periodic switchover at regular intervals as in theprior systems referred to, the switching rate is close to thefundamental or a subharmonic of a low-frequency modulation of the signalwave to be monitored. This results in a stroboscopic effect leading to aslow variation in the measured parameter (amplitude or phase) at a beatfrequency corresponding to the difference between the switchingfrequency and the modulating frequency or its subharmonic. The system,reacting to this beat frequency, introduces unnecessary and undesirablecorrective measures.

Another object of my invention, therefore, is to provide means in such asystem for suppressing the abovedescribed stroboscopic effect.

Both these objects are realized, pursuant to my present invention, bythe provision of a timing circuit which establishes the alternatingcalibration and testing intervals and whose operation is subject tomodification by a voltage-responsive control circuit. By applying tothis control circuit an input voltage depending upon the magnitude ofthe incoming signal voltage, as determined by a conventional thresholddevice such as a differential amplifier, I can slow down the rate ofswitchover (or, as a limiting case, arrest the timer with the system inits test position) as long as the integrated signal amplitude does notrise to a predetermined minimum level. On the other hand, the switchoverrate could be continually changed in a random or quasi-random fashionwithin certain limits (e.g., between 0.5 and 1.5 cycles per second) byapplying to the control circuit an irregularly varying input voltagederived, for example,from a white-noise generator.

The above and other features of my invention will be described in detailhereinafter with reference to the accompanying drawing in which:

FIG. 1 is an overall block diagram of a first embodiment;

FIG. 2 is a similar block diagram of a second embodiment;

FIG. 3 is a more detailed circuit diagram for the embodiment of FIG. 1;

FIG. 4 shows a partial modification of the circuit diagram of FIG. 3;

FIG. 5 illustrates another partial modification of that circuit diagram;and

FIG. 6 is a set of graphs relating to the operation of the system ofFIG. 3.

In FIG. 1 I have shown a system broadly similar to that of FIG. 5 ofeach of my two above-identified prior patents, the same referencenumerals having been used where applicable. Thus, in the illustratedtest position of that system, a signal voltage of predeterminedfrequency f is fed via a switch 514 to a selector network 10 workinginto an adjustable amplifier 505 whose output is delivered to a detector506; another switch 520, ganged with switch 514, transmits the detectoroutput to a voltmeter 507 and to a smoothing capacitor 521 in paralleltherewith.

In the alternate position of switches 514 and 520, a locally generatedreference oscillation of fixed frequency f is applied to the input offrequency selector 10 while the output of detector 506 goes to a storagecapacitor 522 forming part of a sensing circuit and controlling the gainof amplifier 505. (Instead of a variablegain amplifier, a variableattenuator could be used as described in the above-identifiedapplication Ser. No. 737,544.) The periodic reversal of switches 514 and520 is controlled by a timer 519.

In accordance with the present invention, I connect a threshold device11 to the output of detector 506, in parallel with meter 507. Duringeach calibrating interval the timer 519, controlled by the thresholddevice 11, operates at its normal rate (e.g., l c.p.s.) as long as thesignal voltage as integrated by the detector equals or exceeds apredetermined minimum level. Whenever the signal voltage falls belowthat level, device 11 trips the timer 519 to lengthen the testinginterval by, say, a factor of without affecting the length of thecalibration interval.

FIG. 2 shows a similar system, differing from that of FIG. 1 by thesubstitution of an adjustable phase shifter 505' and a phase comparator506 for the variable-gain amplifier 505 and the detector 506. Withswitches 514 and 520 in their illustrated testing position, a unipolarvoltage proportional to the phase difference between signal wave f (asmodified by phase shifter 505) and reference wave f., is delivered bycomparator 506' to meter 507 here serving as a phase indicator. In thecalibration position, capacitor 522 accumulates a charge whosemagnitude, if deviating from a standard, readjusts the phase shifter 505in the same manner as amplifier 505 of FIG. 1 is adjusted by ananalogous deviation, as more fully described in application Ser. No.737,544.

In FIG. 2 the threshold device 11, responding to a parameter (amplitude)different from the one to be measured (phase), is connected to thetransmission circuit between switches 514 and 520 at a location ahead ofphase comparator 506', i.e. in the output of phase shifter 505. Again, adrop in signal voltage below a predetermined level modifies theoperation of timer 519 as described above.

FIG. 3 shows details of circuit 10, threshold device 11 and timer 519 inthe system of FIG. 1. Circuit 10 includes a number of elements shown inFIG. 5 of my two prior patents, i.e. an input potentiometer 501, anamplifier S02 and a mixer 503 which receives a locally generated testfrequency f, from an adjustable oscillator 509 and supplies a beatfrequency f =fi f to a band-pass filter 504 centered on that beatfrequency. The filter output energizes a transformer 515 with twosecondary windings having output leads 517 and 518; these secondarywindings are ganged with the slider of potentiometer 501 forcompensatory adjustment of their output voltages. Test frequency f, isalso delivered to a mixer 511 receiving another locally generatedfrequency f from an oscillator 512 to synthesize therefrom the referencefrequency f, =f, f,

The switches 514 and 520 of FIG. 1 have been represented in FIG. 3 bypaired armatures of respective relay pairs 514', 514" and 520' and 520"connected, in series with another relay pair 516, 516", in the collectorleads of two NPN transistors 519a, 5l9b forming respective sections ofan astable multivibrator which is part of the timer-519. The collectorof transistor 519a and the base of transistor 51% are cross-coupled viaa condenser 12 which forms part of an R/C circuit also including aresistor 13 connected between that base and a positive bus bar 14. Asimilar reactive network comprises a condenser 15 coupling the collectorof transistor 519b to the base of transistor 519a, a highohmic resistor16 between that base and bus bar 14, and a low-ohmic resistor 17connected across resistor 16 in series with a normally saturated PNPtransistor 18. As is well known, the time constant of each of these R/Cnetworks is determined by the magnitudes of the capacitances l2, l5 andof the associated charging resistances 13, 16 and 17, the time constantof multivibrator section 519a being thus sharply increased whenever thetransistor 18 is cut off to disconnect the shunt path through low-ohmicresistor 17. If, with equal capacitances, the resistances 13, 16 and 17are in the ratio of 1:1000zl0 (e.g., of magnitudes I [(0, 1 MO and l KO,respectively), a similar ratio will exist between the invariable timeconstant of the right-hand half and the two alternate time constants ofthe left-hand half of the multivibrator. This has been illustrated inFIG. 6 where graphs (a) and (b) show the periods T and T of current flowi and i through transistors 519a and 519b, respectively, with transistor18 conducting to lower the time constant of the left-hand multivibratorsection; graphs (c) and (d) show the two currents with transistor 18 cutoff, it being noted that the period T of conductivity of transistor 519ahas remained unchanged but that the corresponding period of transistor519b has been lengthened to a value T equaling a multiple of period T Inkeeping with the example previously given, we may assign to theseperiods the values T =0.l sec, T =0.9 sec and T z 100 sec.

The conduction of ancillary transistor 18 is controlled by adifferential amplifier 19 which forms part of threshold device 11 andhas its noninverting input connected to a potentiometer 20 between busbar 14 and ground. The inverting input of amplifier 19 is connected inparallel with the input of meter 507 so as to be energized from theoutput of amplifier 505 via detector 506 by way of closed contacts ofrelay 520" whenever current i flows through transistor 519k. Under theseconditions, relays 514" and 516" are also operated to attract theirarmatures and complete the circuits from the source of input voltage fto potentiometer 501 and from transformer output lead 517 to the inputof amplifier 505 whereby testing may proceed as described in my priorpatents referred to. During this period of conductivity of transistor519b, condenser progressively charges through resistors 16 and 17 todrive the base of transistor 519a sufficiently positive to trip themultivibrator 519; transistor 519a now conducts, applies a negativecut-off pulse to the base of transistor 519b through coupling condenser12, and energizes the relays 514', 516' and 520' in lieu of theircompanion relays to switch the system to its calibrating position. Thissituation endures until, after an interval T determined by the timeconstant of network 12, 13, condenser 12 has recharged sufficiently torestore the multivibrator to its previous condition, thereby driving thebase of transistor 519a negative and commencing another charging periodfor condenser 15.

If the output voltage of detector 506 stored on capacitor 521 dropsbelow its critical value, the output of differential amplifier 19 goespositive and blocks the ancillary transistor 18 so that condenser 15 cancharge only through resistor 16 during the next cycle. This accounts forthe lengthening of the test interval from T to T as shown in FIG. 6.

Since, with the illustrated pairing of relays 514', 514" etc., theirarmatures need not be provided with back contacts, these armatures andtheir front contacts can be designed as sensitive reed switches. As longas bus bar 14 remains energized, one contact of each pair always makeswhile the other one breaks: if power fails, all relay contacts are open.

The emitters of transistors 519a and 51% are shown grounded through acommon resistor 21 to help maintain the astable condition of themultivibrator.

In FIG. 4 a modification of the circuit of FIG. 3 is shown (illustratedonly in part) in which an ancillary NPN transistor 18 is connectedbetween ground and the base of transistor 519a. As long as the signal f(FIG. 3) comes in strong, transistor 18' is cut off and themultivibrator 519 operates at a switchover rate determined by the timeconstants of reactive networks 12, 13 and 15, 17 (the high-ohmicresistor 16 having been omitted in this modified system). When thesignal falls below the critical level, transistor 18' is turned on toprevent the charging of condenser to a potential which would drive thetransistor 519a into conduction; thus, multivibrator section 51% remainscontinuously energized with operation of relays 514", 516" and 520"(FIG. 3) by the current i The alternation of testing and calibration isresumed whenever the measured signal voltage returns to its preset levelselected with the aid of potentiometer 20.

It will be apparent that the circuit arrangement of FIG. 3 or FIG. 4could be used in either of the two systems shown in FIGS. 1 and 2.

FIG. 5 shows another partial modification of the system of FIG. 3wherein, however, the sensing circuit 11 has been replaced by awhite-voise generator 22 working into an integrating network 23 in theinput of an NPN transistor 18" bridged across part of resistor 16. Theintegrated output of generator 22 varies the conductivity of transistor18" in an irregular pattern,

between limits corresponding to a range of time constants equivalent toa conductivity period for multivibrator half 51% (not shown in FIG. 5)between a lower limit of, say, 0.5T and an upper limit of, say, 1.5Tbased on the value previously assumed for that period. It will beunderstood that the control circuit of FIG. 5 could be combined withthat of FIG. 3 or FIG. 4, as by connecting transistor 18 in cascade withtransistor 18" or including transistor 18' in the network of FIG. 5. Insuch a combined system, the test intervals would fluctuate about arelatively short mean period in the presence of an input signal andwould be extended to a relatively long period (possibly with completesuppression of the calibration interval) in the absence of such signal.

It will be noted that, whenever the calibration phase is suppressed foran extended or indefinite period, the system retains its most recentadjustment.

The white-noise generator 22 of FIG. 5 could be replaced by a source ofpseudo-random voltage generated under the control of a programmer in anirregular but recurrent pattern. Such random or quasirandom modulationcould also be imparted to the calibration phase instead of, or inaddition to, the testing phase as specifically described above. I

I claim:

1. A circuit arrangement for giving a calibrated indication of aparameter of an alternating signal voltage of predetermined frequency,comprising:

circuit means connectable to a source of signal voltage to be measured;

local oscillator means for generating a reference oscillation of saidpredetermined frequency; indicator means for measuring said parameter;sensing means for varying the transmission characteristic of saidcircuit means; automatic switchover means for alternately establishing atest position connecting said indicator means through said circuit meansto said source and a calibrating position connecting said sensing meansthrough said circuit means to said oscillator means; and

threshold means connected to said circuit means for ascertaining themagnitude of said signal voltage in said test position and reducing therate of operation of said switchover means upon said magnitude fallingbelow a predetermined level.

2. A circuit arrangement for giving a calibrated indication of aparameter of an alternating signal voltage of predetermined frequency,comprising:

circuit means connectable to a source of signal voltage to be measured;

local oscillator means for generating a reference oscillation of saidpredetermined frequency;

indicator means for measuring said parameter;

sensing means for varying the transmission characteristic of saidcircuit means;

automatic switchover means for alternately establishing a testpositionconnecting said indicator means through said circuit means to. saidsource and a calibrating position connecting said sensing means throughsaid circuit means to said oscillator means; and

threshold means connected to said circuit means for ascertaining themagnitude of said signal voltage in said test position and arresting theoperation of said switchover means in said test position upon saidmagnitude falling below a predetermined level.

3. A circuit arrangement for giving a calibrated indication of aparameter of an alternating signal voltage of predetermined frequency,comprising:

circuit means connectable to a source of signal voltage to be measured;

local oscillator means for generating a reference oscillation of saidpredetermined frequency;

indicator means for measuring said parameter;

sensing means controlling said circuit means for varying thetransmission characteristic thereof;

astable multivibrator means provided with reactive circuitry forestablishing a testing interval and a calibration interval alternatingin a rhythm determined by the time constant of said circuitry;

switch means controlled by said multivibrator means for connecting saidindicator means through said circuit means to said source during saidtesting interval and for connecting said sensing means through saidcircuit means to said oscillator means during said calibration interval;and i a generator of irregularly varying control voltages I coupled tosaid reactive circuitry for altering said time constant.

4. A circuit arrangement for giving a calibrated indication of aparameter of an alternating signal voltage of predetermined frequency,comprising;

circuit means connectable to a source of signal voltage to be measured;i local oscillator means for generating a reference oscillation of saidpredetermined frequency; indicator means for measuring said parameter;sensing means controlling said circuit means for varying thetransmission characteristic thereof; astable multivibrator meansprovided with reactive circuitry for establishing a testing interval anda calibration interval alternating in a rhythm determined by the timeconstant of said circuitry; switch means controlled by saidmultivibrator means for connecting said indicator means through'saidcircuit means to said source during said testing interval and forconnecting said sensing meansthrough said circuit means to saidoscillator means during said calibration interval;

voltage-responsive control means coupled to said reactive circuitry foraltering said time constant; and I a generatorof input voltage includingthreshold means connected to said circuit means for ascertaining themagnitude of said signal voltage in said test position and actuatingsaid control means to lower said time constant upon said magnitudefalling below a predetermined level.

5. A circuit arrangement as defined in claim 4 wherein said indicatormeans comprises an amplitude meter.

6. A circuit arrangement as defined in claim 4 wherein said indicatormeans comprises a phase meter.

7. A circuit arrangement as defined in claim 6 wherein said circuitmeans includes a phase shifter controlled by said sensing means and aphase comparator following said phase shifter, said switch meanscomprising contacts interposed between said comparator and said meter,said threshold means comprising a voltage detector connected to saidcircuit means ahead of said comparator.

8. A circuit arrangement as defined in claim 4 wherein said switch meanscomprises at least one pair of relays in series with respective sectionsof said multivibrator means, said relays being provided with pairedcontacts respectively making and breaking upon energization of either ofsaid sections.

1. A circuit arrangement for giving a calibrated indication of aparameter of an alternating signal voltage of predetermined frequency,comprising: circuit means connectable to a source of signal voltage tobe measured; local oscillator means for generating a referenceoscillation of said predetermined frequency; indicator means formeasuring said parameter; sensing means for varying the transmissioncharacteristic of said circuit means; automatic switchover means foralternately establishing a test position connecting said indicator meansthrough said circuit means to said source and a calibrating positionconnecting said sensing means through said circuit means to saidoscillator means; and threshold means connected to said circuit meansfor ascertaining the magnitude of said signal voltage in said testposition and reducing the rate of operation of said switchover meansupon said magnitude falling below a predetermined level.
 2. A circuitarrangement for giving a calibrated indication of a parameter of analternating signal voltage of predetermined frequency, comprising:circuit means connectable to a source of signal voltage to be measured;local oscillator means for generating a reference oscillation of saidpredetermined frequency; indicator means for measuring said parameter;sensing means for varying the transmission characteristic of saidcircuit means; automatic switchover means for alternately establishing atest position connecting said indicator means through said circuit meansto said source and a calibrating position connecting said sensing meansthrough said circuit means to said oscillator means; and threshold meansconnected to said circuit means for ascertaining the magnitude of saidsignal voltage in said test position and arresting the operation of saidswitchover means in said test position upon said magnitude falling belowa predetermined level.
 3. A circuit arrangement for giving a calibratedindication of a parameter of an alternating signal voltage ofpredetermined frequency, comprising: circuit means connectable to asource of signal voltage to be measured; local oscillator means forgenerating a reference oscillation of said predetermined frequency;indicator means for measuring said parameter; sensing means controllingsaid circuit means for varying the transmission characteristic thereof;astable multivibrator means provided with reactive circuitry forestablishing a testing interval and a calibration interval alternatingin a rhythm determined by the time constant of said circuitry; switchmeans controlled by said multivibrator means for connecting saidindicator means through said circuit means to said source during saidtesting interval and for connecting said sensing means through saidcircuit means to said oscillator means during said calibration interval;and a generator of irregularly varying control voltages coupled to saidreactive circuitry for altering said time constant.
 4. A circuitarrangement for giving a calibrated indication of a parameter of analternating signal voltage of predetermined frequency, comprising;circuit means connectable to a source of signal voltage to be measured;local oscillator means for generating a reference oscillation of saidpredetermined frequency; indicator means for measuring said parameter;sensing means controlling said circuit means for varying thetransmission characteristic thereof; astable multivibrator meansprovided with reactive circuitry for establishing a testing interval anda calibration interval alternating in a rhythm determined by the timeconstant of said circuitry; switch means controlled by saidmultivibrator means for connecting said indicator means through saidcircuit means to said source during said testing interval and forconnecting said sensing means through said circuit means to saidoscillator means during said calibration interval; voltage-responsivecontrol means coupled to said reactive circuitry for altering said timeconstant; and a generator of input voltage including threshold meansconnected to said circuit means for ascertaining the magnitude of saidsignal voltage in said test position and actuating said control means tolower said time constant upon said magnitude falling below apredetermined level.
 5. A circuit arrangement as defined in claim 4wherein said indicator means comprises an amplitude meter.
 6. A circuitarrangement as defined in claim 4 wherein said indicator means comprisesa phase meter.
 7. A circuit arrangement as defined in claim 6 whereinsaid circuit means includes a phase shifter controlled by said sensingmeans and a phase comparator following said phase shifter, said switchmeans comprising contacts interposed between said comparator and saidmeter, said threshold means comprising a voltage detector connected tosaid circuit means ahead of said comparator.
 8. A circuit arrangement asdefined in claim 4 wherein said switch means comprises at least one pairof relays in series with respective sections of said multivibratormeans, said relays being provided with paired contacts respectivelymaking and breaking upon energization of either of said sections.